1. Field of the Invention
This invention relates to detection and interruption of currents and circuits experiencing arc faults. More particularly, it relates to an arc fault detector and a circuit breaker incorporating such an arc fault detector which minimize the effects of other phenomena such as the burnout of tungsten bulbs, which can falsely provide an indication of an arc fault.
2. Background Information
Arc faults can occur in electrical systems for instance between adjacent bared conductors, between exposed ends of broken conductors, at a faulty connection, and in other situations where conducting elements are in close proximity. Arc faults in ac systems can be intermittent as the magnetic repulsion forces generated by the arc current force the conductors apart to extinguish the arc. Mechanical forces then bring the conductors together again so that another arc is struck.
Arc faults typically have high resistance so that the arc current is below the instantaneous or magnetic trip thresholds of conventional circuit breakers. Also, the intermittent nature of an arc fault can create an average RMS current value which is below the thermal threshold for such circuit breakers. Even so, the arcs can cause damage or start a fire if they occur near combustible material. It is not practical to simply lower the pickup currents on conventional circuit breakers as there are many typical loads which draw similar currents, and would therefore, cause nuisance trips.
Much attention has been directed toward trying to distinguish arc currents from other intermittent currents. It has been recognized that arc currents generate a step increase in current when the arc is struck. However, many typical loads generate a similar step increase, such as for instance when the device is turned on. In many instances, the step increases generated by these loads are singular events while an arc fault generates a series of step increases. The arc fault detector described in U.S. Pat. No. 5,224,006 counts the step increases in current and generates a trip signal if a selected number of step increases occur within a given interval. However, there are loads, such as a solid state dimmer switch with the firing angle phased back substantially, which also generate repetitive step increases in current. This problem is addressed by the arc fault detector in U.S. Pat. No. 5,691,869 in which the arc current is passed through a bandwidth limited filter which generates pulses having an amplitude proportional to the step increases. An arc indication is generated when a time attenuated accumulation of these pulses reaches a predetermined value. Thus, a few very large magnitude step increases within a period of time, or a larger number of more modest step increases within a similar time period, generate a trip signal. The trip level can be set so that the cyclic pulses generated by a dimmer do not generate the time attenuated accumulation which reaches the trip level.
There is at least one arc condition which can occur in a protected circuit to which it is desired that the arc fault circuit not respond. This is an arc created by the burnout of a tungsten filament such as in a light bulb. When the filament burns through, a small gap is created between the burned out ends of the filament. An arc is struck across this gap and can quickly envelop the entire filament so that it extends between the two conductors thereby drawing a very large arc current. In order to terminate this arc, tungsten bulbs are provided with a small fuse in the base. Even so, burnout of the filament and blowing of the fuse results typically in a pair of current pulses of opposite polarity. This pair of pulses can be of sufficient magnitude that the threshold value of the time attenuated accumulation of pulses in the circuit breaker described in U.S. Pat. No. 5,691,869 is exceeded and the circuit breaker is tripped. This is considered a nuisance trip as the fuse has interrupted the arc.
Tungsten filament bulbs can also generate false trips when used with a dimmer. As mentioned, a dimmer which is phased back can generate repetitive step increases in current on each half cycle. As also discussed, the circuit breaker can be set so that the threshold of the time attenuated accumulation of pulses generated by the dimmer do not reach the trip level with normal loads. However, when a tungsten filament lamp is first turned on, the cold filament has a very low resistance and can draw up to fifteen times normal current. This can result in a nuisance trip when a tungsten lamp controlled by a dimmer switch is first turned on.
Commonly owned U.S. patent application Ser. No. 08/939,976 filed on Sep. 29, 1997, addresses the problems created by a tungsten filament bulb by disclosing an arc fault detector in which a zener diode places amplitude limits on pulses having an amplitude proportional to the magnitude of step increases in current in the protected circuit so that the time attenuated accumulation of these pulses does not reach the trip level based upon a few very large pulses which can be generated by burnout of a tungsten lamp or turn on of a tungsten lamp controlled by a dimmer switch. In particular, the zener diode clips the pulses generated in response to the step increases in current in the protected circuit before the time attenuated accumulation of the pulses. As true arc faults will continue to strike at a random rate, and therefore raise the time attenuated accumulation of pulses to the trip threshold, false trips due to burnout of a tungsten lamp or turn on of a cold tungsten bulb are avoided. While the arc detector described in the cited patent application reduces false trips due to tungsten lamp burnout and turn on of dimmer controlled tungsten bulbs, there is room for improvement.
There is a need for an arc fault detector and circuit breaker incorporating such a detector which can respond faster to true arc faults, yet not falsely trip on other phenomena such as burnout of a tungsten bulb or turn on of a tungsten bulb controlled by a dimmer.
There is a need for achieving such a result with a simple, reliable and low cost detector.
In particular there is a need for such apparatus which can respond sooner to step increases in current in a protected circuit, whether small or large and yet discriminate against current discontinuities caused by tungsten bulbs.